The invention relates to detecting fractures present in underground earth formations having a borehole drilled therethrough.
There are several reasons for the desirability of detecting fractures in oil prospecting, the most important of which being that fracture zones have a high chance of constituting hydrocarbon production zones. The fracture system usually provides a high permeability path and consequently constitutes an effective draining mechanism through reservoir rock which is generally dense and of low permeability.
Many currently known tools are capable, after suitable interpretation of their measurements, of giving indications of the presence of fractures but not in a really satisfactory manner.
Of the available tools, particular attention has been given to sonic tools which transmit and receive sound waves. Tests have been performed in order to evaluate the possibility of practical fracture detection using such a tool. In this respect, reference is made to the following publications: R. L. Morris, D. R. Grine and T. E. Arkfeld, 1964 "The use of compressional and shear acoustic amplitudes for the location of fractures", J. Pet. Tech., 16, 623-635; E. A. Koerperich, 1978 "Investigation of Acoustic Boundary Waves and Interference Patterns as Techniques for Detecting Fractures", J. Pet. Tech., 30, 1199-1207; and F. L. Paillet, 1980 "Acoustic Propagation in the Vicinity of Fractures which intersect a Fluid-filled Borehole", Proceedings SPWLA Symposium, 1980, paper DD.
These previously known methods make heavy use of a qualified expert interpreting a suitable representation of the measurements performed by a particular sonic tool. The person skilled in the art will readily recognize the drawbacks of such an approach.